1. Field of the Invention
The present invention relates to a non-volatile semiconductor memory device that compares currents flowing in a reference memory cell and a main memory cell by a sense amplifier to thereby perform data sensing, and more specifically, to a non-volatile semiconductor memory device in which threshold voltages of a reference memory cell and a main memory cell are adjusted.
2. Description of the Related Art
Various kinds of electrically batch-erasable non-volatile semiconductor memory devices having, for example, an EEPROM cell (hereinafter referred to as flash memories) have been developed. In Jpn. Pat. Appln. KOKAI Publication No. 2004-103211, or B. Pathank et al., A 1.8V 64 Mb 100 MHz Flexible Read While Write Flash Memory, 2001, IEEE International Solid-State Circuits Conference, a NOR type flash memory of a current comparison type sensing system is disclosed. Read and verify operations in the flash memory of this system are performed by comparing currents flowing in a selected memory cell and a reference memory cell by means of a sense amplifier.
In the current comparison type sensing system, there are known two kinds, that is, an Iref direct system in which a reference current Iref is directly supplied to an input terminal of a sense amplifier, and an Iref mirror system in which a reference current Iref is supplied to an input terminal of a sense amplifier via a current mirror circuit.
In a memory of the Iref direct system, plural reference memory cells are used. Threshold voltages of the respective reference memory cells are not uniform, but uneven. When the threshold voltages of the reference memory cells fluctuate, a threshold voltage of a main memory cell also fluctuates.